MySQL how to get avg of values into specific range

Question

I have the following table data:

value
1
5
10.5
12
36

I want to map these values into:

range      avg
0-21       (1 + 5 + 10.5 + 12) / 4
21.001-34  0
34.001-64  36
64 +       0

Basically map each value to ranges and calculate the avg of all values in each range.

I tried to do:

select 
case
when avggeneral between 0 and 21 then ' 0-21'
when avggeneral between 21.00001 and 34 then ' 21-34'
when avggeneral between 34.00001 and 64 then ' 34-64'
else '64+'
end as 'range',
AVG(*) as 'avg'
from table

but this doesn't work... I don't know how to make the static ranges...

How can I do that in MySQL?

Methology should be: 1. Map values into these groups ( 0-21, 21-34 etc..) 2. calulate AVG in each group.

Desired output for above example is this:

range      avg
0-21       7.125
21.001-34  0
34.001-64  36
64 +       0

The range column is static. Always with 5 rows. The avg column is dynamic.. the values there are the actual AVGs of value column.

Answer 1

You could build a list of ranges using UNION ALL and LEFT JOIN with it:

SELECT CONCAT(IFNULL(ranges.min, '∞'), '-', IFNULL(ranges.max, '∞')) AS `range`, avg(value) AS avg
FROM (
    SELECT 0 AS min, 21 AS max UNION ALL
    SELECT 21, 34 UNION ALL
    SELECT 34, 64 UNION ALL
    SELECT 64, NULL
) AS ranges
LEFT JOIN t ON (ranges.min IS NULL OR value >= ranges.min) AND
               (ranges.max IS NULL OR value <  ranges.max)
GROUP BY ranges.min, ranges.max

Note that the above query will put 20.9999 inside [0-21) and 21.0000 inside [21-34) range.